The overall aims of the studies proposed here are to use the in utero human/sheep xenograft model of human hematopoiesis to delineate 1) the parameters that promote the long term, stable engraftment of donor human fetal and adult hematopoietic stem cells (HSC) with high efficiency without cytoablation and without GVHD, and 2) the conditions that permit significant expression and differentiation of donor cells sufficiently early in gestation to protect the patient from the adverse effects of the disease in utero. Transplantation of human HSC in sheep represents an ideal and unique model for experimental approaching the question of the feasibility of the in utero approach to HSC transplantation. We plan to use this mode to determine the optimal concentration of human fetal HSC that would give rise to therapeutic levels of donor cell activity. Because the use of fetal tissue in human presents significant problems, we will develop the strategy for achieving significant donor cell engraftment with multilineage expression using human cord blood and adult peripheral blood or bone marrow stem cells. Preliminary studies indicate that purified fractions of post-natal HSC represent a viable source without development of GVHD. We anticipate that in all cases the induction of tolerance to donor HSC will permit significant boosting of donor cell levels by the infusion of donor cells soon after birth. It is hoped that the in utero HSC transplantation procedures developed here will permit the treatment of some lymphohematopoietic and metabolic disorders in utero before the disease has had a chance to clinically compromise the baby.